Progress Reports are publications sent out before the convention, telling people what is happening with the convention planning and what they can expect when they arrive at the convention. They can be as small as postcards or one-page flyers and as big as booklets.
It is fairly traditional for the last PR (before the convention) to contain travel details on how to find the site, plus a reminder of things like whether the member needs to bring photo ID or the mailing label from the PR to "prove" who they are. It may also remind the member to bring other items (e.g. formal attire for the banquet, a cake for the charity cake stall, a robot for the robot battles, fanzines for the fanzine library etc.)
Remember the Seven Things!
Many people opt out of paper progress reports, preferring to read them on the convention website as PDFs. Send out an email with a link when they are published. Because that makes them public documents, conventions generally ask members if they wish to opt out of any membership lists they might contain. If the website has a membership list, there is little point repeating it in the PR.
It is tempting to abandon PRs entirely, relying on social media, but this is unwise as many people refuse to use Facebook and Twitter, and indexing and search tools are poor. At the very least have an update area of your website containing the vital PR information.